diff --git a/curve25519-dalek/Cargo.toml b/curve25519-dalek/Cargo.toml
index e285c93..ef5dc81 100644
--- a/curve25519-dalek/Cargo.toml
+++ b/curve25519-dalek/Cargo.toml
@@ -56,25 +56,26 @@ subtle = { version = "2.3.0", default-features = false }
 serde = { version = "1.0", default-features = false, optional = true, features = ["derive"] }
 zeroize = { version = "1", default-features = false, optional = true }
 
-# Betrusted/Precursor dependency set
-[target.'cfg(curve25519_dalek_backend = "u32e_backend")'.dependencies]
-engine25519-as = {git = "https://github.com/betrusted-io/engine25519-as.git", rev = "d249c967556b02ab5439eacb5078fa00c60b93d6", default-features = false, features = []}
-engine-25519 = { git = "https://github.com/betrusted-io/xous-engine-25519.git", rev = "63d3d1f30736022e791deaacf4dd62c00b42fe2e"}
-utralib = {version = "0.1.0"} # this is bogus -- must be patched in the invoking build environment TODO: if this builds, it seems a crate has been released as version 0.1.23 that could replace this
+# Betrusted/Precursor dependency set, enabled by backend_u32e feature
+engine25519-as = {git = "https://github.com/betrusted-io/engine25519-as.git", rev = "d249c967556b02ab5439eacb5078fa00c60b93d6", default-features = false, features = [], optional = true}
+engine-25519 = { git = "https://github.com/betrusted-io/xous-engine-25519.git", rev = "63d3d1f30736022e791deaacf4dd62c00b42fe2e", optional = true}
+utralib = {version = "0.1.0", optional = true} # this is bogus -- must be patched in the invoking build environment TODO: if this builds, it seems a crate has been released as version 0.1.23 that could replace this
 
 [target.'cfg(target_arch = "x86_64")'.dependencies]
 cpufeatures = "0.2.6"
 
-[target.'cfg(curve25519_dalek_backend = "fiat")'.dependencies]
-fiat-crypto = { version = "0.2.1", default-features = false }
+fiat-crypto = { version = "0.2.1", default-features = false , optional = true}
 
 [features]
 default = ["alloc", "precomputed-tables", "zeroize"]
 alloc = ["zeroize?/alloc"]
 precomputed-tables = []
 legacy_compatibility = []
+backend_fiat = ["dep:fiat-crypto"]
+backend_u32e = ["dep:engine25519-as","dep:engine-25519","dep:utralib"]
 group = ["dep:group", "rand_core"]
 group-bits = ["group", "ff/bits"]
 
-[target.'cfg(all(not(curve25519_dalek_backend = "fiat"), not(curve25519_dalek_backend = "serial"), target_arch = "x86_64", not(curve25519_dalek_backend = "u32e_backend")))'.dependencies]
+# TODO: clean this up as you can't select deps like this
+[target.'cfg(all(not(curve25519_dalek_backend = "fiat"), not(curve25519_dalek_backend = "serial"), target_arch = "x86_64"))'.dependencies]
 curve25519-dalek-derive = { version = "0.1", path = "../curve25519-dalek-derive" }
diff --git a/curve25519-dalek/build.rs b/curve25519-dalek/build.rs
index 055cccd..bda1cd1 100644
--- a/curve25519-dalek/build.rs
+++ b/curve25519-dalek/build.rs
@@ -12,6 +12,12 @@ enum DalekBits {
     Dalek64,
 }
 
+macro_rules! build_debug {
+    ($($tokens: tt)*) => {
+        println!("cargo:warning={}", format!($($tokens)*))
+    }
+}
+
 fn main() {
     let target_triplet = std::env::var("TARGET").unwrap();
     let platform = platforms::Platform::find(&target_triplet).unwrap();
@@ -21,6 +27,8 @@ fn main() {
         Ok("64") => DalekBits::Dalek64,
         _ => deterministic::determine_curve25519_dalek_bits(),
     };
+    build_debug!("CARGO_CFG_CURVE25519_DALEK_BITS: {:?}", std::env::var("CARGO_CFG_CURVE25519_DALEK_BITS").as_deref());
+    build_debug!("curve25519_dalek_bits {:?}", curve25519_dalek_bits);
 
     match curve25519_dalek_bits {
         DalekBits::Dalek64 => println!("cargo:rustc-cfg=curve25519_dalek_bits=\"64\""),
@@ -77,6 +85,7 @@ fn main() {
                 },
             }
         };
+    build_debug!("curve25519_dalek_backend {:?}", curve25519_dalek_backend);
     println!("cargo:rustc-cfg=curve25519_dalek_backend=\"{curve25519_dalek_backend}\"");
 }
 
diff --git a/curve25519-dalek/src/backend/serial/u32/constants.rs b/curve25519-dalek/src/backend/serial/u32/constants.rs
index f55a822..c4fc350 100644
--- a/curve25519-dalek/src/backend/serial/u32/constants.rs
+++ b/curve25519-dalek/src/backend/serial/u32/constants.rs
@@ -69,7 +69,7 @@ pub(crate) const SQRT_M1: FieldElement2625 = FieldElement2625::from_limbs([
 
 /// `APLUS2_OVER_FOUR` is (A+2)/4. (This is used internally within the Montgomery ladder.)
 /// The u32e backend uses hardware acceleration for this.
-#[cfg(not(curve_dalek_backend = "u32e_backend"))]
+#[cfg(not(curve25519_dalek_backend = "u32e_backend"))]
 pub(crate) const APLUS2_OVER_FOUR: FieldElement2625 =
     FieldElement2625::from_limbs([121666, 0, 0, 0, 0, 0, 0, 0, 0, 0]);